This invention relates to a metal casting produced by a lost foam process utilizing a vaporizable pattern and having a bore surrounded by pore-free metal, such as an engine block casting having a combustion cylinder. More particularly, this invention relates to a foundry chill device receivable in the pattern bore to accelerate solidification of the cast metal thereabout, without interfering with venting pattern decomposition vapors, to reduce porosity in the casting region about the bore.
In a lost foam casting process, a vaporizable polymeric pattern is embedded in a gas-permeable mold formed of unbonded refractory particles, such as lake sand. The pattern is sized and shaped for duplication by the metal to produce a casting of a desired design. The pattern is preferably coated with a thin, vapor-permeable refractory layer to improve casting properties. Molten metal poured in the mold decomposes and replaces the pattern. Pattern decomposition vapors that form during casting vent through the coating into the mold. Vapors that fail to vent become entrapped in the metal and form pores in the product casting. Pores may also form as the result of shrinkage of the metal during solidification.
It has been proposed to cast an engine block for an automotive internal combustion engine by the lost foam process. The pattern for an engine block casting comprises a plurality of bores for forming combustion cylinder walls. Pore-free metal is required in the region surrounding the combustion cylinder. For casting, the bores are packed with the mold sand to shape the metal. Vapors vent into the packed bore to avoid entrapment. However, there remains a tendency for shrink pores to form in the metal about the combustion cylinder.
U.S. Pat. No. 4,520,858, issued to Ryntz et al in 1985, shows a chill member in combination with a lost foam pattern to reduce shrink porosity. The chill accelerates solidification of adjacent cast metal, whereupon shrinkage is fed by still-molten remote metal. In the described embodiment of the patent, the chill fits intimately and continuously against an external pattern surface. However, this continuous chill blocks vapors, which must vent by an alternate route to avoid entrapment. Development of alternate venting may be particularly difficult for vapors generated at a surface within the pattern, such as a bore.
It is an object of this invention to provide an improved foundry chill for use in combination with a vaporizable pattern in lost foam casting, which chill is locatable adjacent a pattern surface and permits pattern decomposition vapors to vent from the surface during casting to avoid entrapment, while accelerating solidification of replacement metal to reduce shrink porosity in the adjacent casting region.
It is a further object of this invention to provide a foundry chill device receivable in a bore of a vaporizable pattern for casting by the lost foam process, which chill is sized and shaped to allow sufficient pattern-mold contact to vent pattern decomposition vapors generated at the bore surface while increasing thermal conduction from the adjacent cast metal to accelerate solidification and thereby reduce shrink porosity.